PF4/heparin complexes bind B cells in a heparin concentration–dependent manner. Whole blood was incubated with various concentrations of PF4 in combination with UFH, LMWH, or fondaparinux as indicated. Cells were stained with CD19 and KKO and the percentage of B cells binding with KKO under each condition is shown on the y-axis. (A) PF4/heparin binding to B cells is heparin dependent. Whole blood was incubated with a fixed amount of PF4 (25 µg/mL) and various concentrations of UFH (0-5 U/mL). Shown are the mean ± SD for percentage of KKO-positive B cells as a function of UFH concentration from 4 independent experiments. (B) B cell binding of complexes formed with PF4 and UFH, LMWH, or fondaparinux. Whole blood was incubated with a fixed amount of PF4 (25 µg/mL) and various concentrations of UFH, LMWH, or fondaparinux. Shown are the mean ± SD for percentage of KKO-positive B cells as a function of PF4 and UFH, LMWH, and fondaparinux molar ratios from 3 independent experiments. (C) B cell binding of PF4/heparin ULCs increases with PF4/heparin concentrations. Whole blood was incubated with increasing amounts of PF4 (0-25 000 ng/mL) and increasing UFH concentrations (0-5 U/mL). KKO binding for a given PF4 concentration is shown as a function of heparin concentration. Shown are the mean ± SD for percentage of KKO-positive B cells from 3 independent experiments.